Heavy petroleum fractions, such as vacuum gas oil, or even resids such as atmospheric resid, may be catalytically cracked to lighter and more valuable products, especially gasoline. Catalytically cracked gasoline forms a major part of the gasoline product pool in the United States. It is conventional to recover the product of catalytic cracking and to fractionate the cracking products into various fractions such as light gases; naphtha, including light and heavy gasoline; distillate fractions, such as heating oil and Diesel fuel; lube oil base fractions; and heavier fractions.
Where the petroleum fraction being catalytically cracked contains sulfur, the products of catalytic cracking usually contain sulfur impurities which normally require removal, usually by hydrotreating, in order to comply with the relevant product specifications. These specifications are expected to become more stringent in the future, possibly permitting no more than about 300 ppmw sulfur in motor gasolines. In naphtha hydrotreating, the naphtha is contacted with a suitable hydrotreating catalyst at elevated temperature and somewhat elevated pressure in the presence of a hydrogen atmosphere. One suitable family of catalysts which has been widely used for this service is a combination of a Group VIII and a Group VI element, such as cobalt and molybdenum, on a suitable substrate, such as alumina.
In the hydrotreating of petroleum fractions, particularly naphthas, and most particularly heavy cracked gasoline, the molecules containing the sulfur atoms are mildly hydrocracked so as to release their sulfur, usually as hydrogen sulfide.
After the hydrotreating operation is complete, the product may be fractionated, or even just flashed, to release the hydrogen sulfide and collect the now sweetened gasoline. Although this is an effective process that has been practiced on gasolines and heavier petroleum fractions for many years to produce satisfactory products, it does have disadvantages.
Naphthas, including light and full range virgin naphthas, may be subjected to catalytically reforming so as to increase their octane numbers by converting at least a portion of the paraffins and cycloparaffins in them to aromatics. Fractions to be fed to catalytic reforming, such as over a platinum type catalyst, also need to be desulfurized before reforming because reforming catalysts are generally not sulfur tolerant. Thus, naphthas are usually pretreated by hydrotreating to reduce their sulfur content before reforming. The octane rating of reformate may be increased further by processes such as those described in U.S. Pat. No. 3,767,568 and U.S. Pat. No. 3,729,409 (Chen) in which the reformate octane is increased by treatment of the reformate with ZSM-5.
Aromatics are generally the source of high octane number, particularly very high research octane numbers and are therefore desirable components of the gasoline pool. They have, however, been the subject of severe limitations as a gasoline component because of possible adverse effects on the ecology, particularly with reference to benzene. It has therefore become desirable, as far as is feasible, to create a gasoline pool in which the higher octanes are contributed by the olefinic and branched chain paraffinic components, rather than the aromatic components. Light and full range naphthas can contribute substantial volume to the gasoline pool, but they do not generally contribute significantly to higher octane values without reforming.
In U.S. Pat. No. 5,346,609 U.S. Pat. No. 5,409,596 and (Ser. No. 08/850,106) we have described a process for effectively desulfurizing catalytically cracked naphthas while maintaining a high octane number. Briefly, the process comprises an initial hydrodesulfurization step which reduces the sulfur to an acceptable level, although at the expense of octane which is restored in a subsequent step by treatment over an acidic catalyst such as one based on ZSM-5, as described in U.S. Pat. No. 5,346,609 and U.S. Pat. No. 5,490,596 (Ser. No. 08/850,106), zeolite beta as described in Ser. No. 07/891,124 (now U.S. Pat. No. 5,413,696) or MCM-22 as described in U.S. Pat. No. 5,352,354. The use of a molybdenum-containing ZSM-5 catalyst is described in Ser. Nos. 08/133,403 and 08/133,942. Reference is made to these disclosures for a detailed description of the problems of cracked naphtha desulfurization as well as of the octane-retention desulfurization processes described in those patents.
Other highly unsaturated fractions boiling in the gasoline boiling range, which are produced in some refineries or petrochemical plants, include pyrolysis gasoline and coker naphtha. Coker naphtha is a fraction which is produced by a coking process, either delayed coking, fluid coking or Contact coking, all of which are well-known processes in the petroleum refining industry. See, for example, Modern Petroleum Technology, Hobson and Pohl (Ed.), Applied Science Publ. Ltd., 1973, ISBN 085334 487 6, pages 283-288, and Advances in Petroleum Chemistry and Refining, Kobe and McKetta, Interscience, N.Y. 1959, Vol. II, pages 357-433, to which reference is made for a description of these processes.
Coker naphtha, being produced by the coking of residual chargestocks, has a high sulfur content, typically at least 1,000 ppmw (0.1 percent by weight) or even higher, for example 5,000 to 10,000 ppmw (0.5 to 1.0 percent) and a low octane number, typically no higher than about 70. It is also unstable and tends to form gums by polymerization of diolefins and other unsaturated species which are present in these thermally cracked products. Although the content of unsaturates is high, with bromine numbers typically in the range of 50 to 80, there is no positive contribution to octane from the unsaturates as they are low octane components. The combination of high sulfur content and low octane makes coker naphtha an unpromising candidate for treatment by the process described in the patents referred to above.
We have found, however, that the use of molybdenum-containing catalysts is favorable for the treatment of coker naphthas, using either medium pore size or large pore size acidic components in the catalysts, especially ZSM-5 and zeolite beta.